Bump third_party/openthread/repo from `f0b6fce` to `518a849` #272

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 25m 37s
Build on Linux (gcc_debug)	50m 4s
Build on Linux (python_lib)	20m 29s
Build on Darwin (clang, python_lib, simulated)	2h 0m 41s
Build on Linux (coverage)	11m 8s
Build on Linux (python lighting-app)	6m 8s
	4h 54m 7s

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